The journalists were released without charges but had their work permits revoked and their equipment confiscated, according to a source at French newspaper Le Monde.

Rémy, who is Africa bureau chief for French daily Le Monde, and Moore, who contributes regularly to the New York Times and AFP, were arrested on Thursday afternoon along with 15 others.

Both have covered the region for years and have won several awards for their reporting and photography.

The journalists were arrested during a sweep for rebels in flashpoint districts of the capital, officials said on Friday, a move likely to further strain tense relations between Burundi and Western donors.

Moise Nkurunziza, deputy police spokesman, said the police had picked up Moore and Rémy during raids in Jabe and Nyakabiga neighbourhoods in Bujumbura on Thursday. Police said a mortar, a Kalashnikov rifle and pistols were also seized during the raid.

Crackdown on press

In a statement on its website, Le Monde had demanded the release of both journalists, saying they were the newspaper's special correspondents in Burundi. A representative from Le Monde told AFP that Rémy and Moore had both been issued visas and entered the country legally.

French ambassador Gerrit Van Rossum visited the pair on Friday at the headquarters of the National Intelligence Service (SNR), where they had been questioned.

On news of their arrest, France's Foreign Minister Laurent Fabius called for their "immediate release", while the British Foreign Office said it was "urgently looking into reports" about the detention of a UK national.

The government has cracked down on the press, forcing independent media to shut down and driving some journalists into exile, including Sonia Rolley from FRANCE 24's sister radio station RFI who was reporting on sexual violence committed by police officers.

More than 400 people have died since the violence erupted and at least 230,000 have fled to neighbouring countries. The UN has warned that the violence could escalate into ethnic killings and mass atrocities.